1. Field of the Invention
The present invention relates generally to X-ray lithography and, more particularly, to a method of correcting for non-uniformities of the X-ray beam which are unique to a particular beamline and its components. The invention also relates to the method of producing a modified heat filter which can accomplish such a correction.
2. Description of the Prior Art
Continuing efforts are being made toward the goal of producing the microscopic features of a new generation of extremely complex integrated electronic circuits such as dynamic random access memories, or DRAMs. By way of example, such anticipated circuits will be capable of containing more than 256 megabytes of information on a microchip sized somewhat larger than a thumbnail. In comparison, current lithographic techniques which use visible light only enable the manufacture of similar sized chips containing up to approximately 64 megabits.
In order to achieve this new plateau of integrated circuit capability, more and more emphasis is being placed on X-ray lithography. Although researchers have experimented with X-ray lithography with such a goal in mind for approximately 20 years, a number of problems with the technique remain before it will be commercially useful.
Optical technology currently being employed and X-ray lithography share some common elements. Both require that manufacturers use "masks" etched with the desired circuit design. Circular silicon wafers, which are eventually separated or diced up into 100 or more chips, are coated with a lithosensitive material, or "resist." Workers then shine ultra violet light or X-rays through the mask onto the wafer, exposing areas of the resist. Washing the wafer with a solvent dissolves the unwanted resist and leaves a copy or pattern of the mask design on the wafer. A finished chip is a sandwich of as many as 20 circuit layers.
As circuit designs become more dense, the wavelength of visible light becomes as troublesome for printing circuits as, by way of analogy, a dull crayon is for drawing narrow lines. Several years ago workers expected the limit of the capability of optical lithography would be to produce circuit lines having a width of approximately 0.5 micron. Four-megabit DRAM's, which can store four million bits of data and are currently the most advanced memory chips on the market, have circuit lines measuring about 0.8 micron in width. But by improving resists and turning to excimer lasers for shorter wavelengths in the deep-ultraviolet range, researchers have further extended optical technology to 0.35 micron which is sufficient to produce a 64-megabit DRAM.
X-rays, given their shorter wavelengths, should work well at 0.25 to 0.2 micron which is the anticipated feature size of 256-megabit chips. Moreover, workers can print crisper lines with X-rays than with visible or ultra violet light because the shorter wavelength means that light stays focused over a longer distance.
A particularly troublesome problem still being faced by researchers of the X-ray lithography technique resides in the observation that each X-ray beamline, including its associated components, has its own unique and permanent signature of non-uniform radiation reaching the photoresist on the target wafer. This is due to imperfections in the collimating mirror, and density variations in the heat filter, beryllium window, and X-ray mask membranes. This signature of non-uniform radiation is undesirably superimposed on all intended radiation employed for producing circuitry on a microchip. This results in imperfections in the circuitry which cannot be tolerated commercially.
It was in light of the prior art as just described that the present invention has been conceived and now reduced to practice.